Great news from Sigma! Finally APS-C users have a tool that is (on paper at least) not only equal, but BETTER than any FF lens available offering the same FOV. While we'll have to wait and see how the lens performs, it is an incredible engineering achievement on one end, and a very welcome creative tool in the bag on the other end...

Following the new announcement, I posted an article on my blog with my reflections on the lens and what it means to APS-C and full-frame users.

Makten wrote:
Why is it crazy or weird? It's a true equivalent of a ~27-53/2.8 on FF, nothing more.

What is pretty impressive is that it has nearly 2 stops more ligth gathering ability compared to a 27-53/2.8 on FF. As in, you can shoot in much lower light with the same depth of field control. Pretty neat in my book

vieri wrote:
What is pretty impressive is that it has nearly 2 stops more ligth gathering ability compared to a 27-53/2.8 on FF. As in, you can shoot in much lower light with the same depth of field control. Pretty neat in my book

That is not correct. You can shoot at higher shutter speeds at the same ISO, but you will also get more noise at the same ISO than with a FF sensor. All other things being equal, you could just up the ISO by a stop for the FF sensor and shoot at the same speed without a noise penalty.

Equivalent lenses wil gather the same amount of light with the same DOF control for different sensor sizes, and that's what ultimately determines noise. That's the whole point of equivalence.

Agree...except for the little issue that it's not being offered in Pentax K mount, which is unfortunate. It's only going to be available for Canon, Nikon and Sigma. Really curious to see what this lens will be priced at as it seems obvious the specifications (moderate wide to normal limited zoom range and size) have been chosen for optimal image quality. My guess is that Sigma expects this lens will at least equal the best primes within it's range.

vieri wrote:
What is pretty impressive is that it has nearly 2 stops more ligth gathering ability compared to a 27-53/2.8 on FF. As in, you can shoot in much lower light with the same depth of field control. Pretty neat in my book

No, what's impressive is that it has THE SAME light gathering ability as a FF 2.8 lens.

The F-stop tells you how much light per square mm the lens will put through. When you multiply that by sensor (or film) area, you get "light gathering ability".

APS has either 2.6 (Canon) or 2.3 (the rest) times smaller area than FF. This means that you need 1+1/3 stop larger apertures on APS to gather the same amount of light per second as on 36x24mm.

F2.8 minus 1+1/3Ev = F1.8.

F-stop tells you "exposure" - which is a per-area measurement. This is a dead metric. It is totally irrelevant in a digital sensor until you know the sensor area.

dcjs wrote:
That is not correct. You can shoot at higher shutter speeds at the same ISO, but you will also get more noise at the same ISO than with a FF sensor. All other things being equal, you could just up the ISO by a stop for the FF sensor and shoot at the same speed without a noise penalty.

Equivalent lenses wil gather the same amount of light with the same DOF control for different sensor sizes, and that's what ultimately determines noise. That's the whole point of equivalence.

And it's a massively flawed argument that has no real bearing on image production, because the equivalence ONLY holds if the sensor technologies in the two compared cameras are identical. It's OK for determining similar depth of field requirements, but saying it's truly equivalent is far more false than it is true.

How do you compare lenses when one person is shooting on a 5D classic vs a D800? The D800 is 1-2 stops better at high ISO, so is an f/2.8 lens shot an a 5D really equivalent to an f/4.5 lens on the D800? No, of course not. Similarly, there are APS-C cameras out there right now that have image quality and noise characteristics very much in line with many full frame cameras of only a generation removed, and even very close to as good as some current gen cameras.

F/1.8 is f/1.8. Any noise considerations are a property of the format and sensor design, not the lens, and so while calculating the shallowest depth of field is a consideration, it's pretty much the ONLY way that the 'equivalent' aperture matters...as soon as you stop down a bit, any advantage is gone (for instance, if one needs the DOF offered by a lens at f/4 on full frame, the APS-C shooter can shoot 1-1/3 stops lower ISO and wider aperture.

This equivalence talk when used in this way isn't about comparing shallowest DOF, it's arguing format size (regardless of lens). I'm going to start coming out and shouting that a 24-70mm f/2.8 is really only an f/4.3 lens, since that's what it's 'equivalent' to on a Phase One IQ2. Who cares what depth of field is on a format that the person buying the lens ISN'T USING?

Jman13 wrote:
And it's a massively flawed argument that has no real bearing on image production, because the equivalence ONLY holds if the sensor technologies in the two compared cameras are identical. It's OK for determining similar depth of field requirements, but saying it's truly equivalent is far more false than it is true.

How do you compare lenses when one person is shooting on a 5D classic vs a D800? The D800 is 1-2 stops better at high ISO, so is an f/2.8 lens shot an a 5D really equivalent to an f/4.5 lens on the D800? No, of course not. Similarly, there are APS-C cameras out there right now that have image quality and noise characteristics very much in line with many full frame cameras of only a generation removed, and even very close to as good as some current gen cameras.

F/1.8 is f/1.8. Any noise considerations are a property of the format and sensor design, not the lens, and so while calculating the shallowest depth of field is a consideration, it's pretty much the ONLY way that the 'equivalent' aperture matters...as soon as you stop down a bit, any advantage is gone (for instance, if one needs the DOF offered by a lens at f/4 on full frame, the APS-C shooter can shoot 1-1/3 stops lower ISO and wider aperture.

Exactly, Jman13. The new lens offers THE SAME depth of field control of a 27-53 f2.8 on full frame, but allows you to shoot with ABOUT TWO STOP LESS LIGHT at the same shutter speed. dcjs can put it however he pleases, but put it however you put it, I'll take it any day of the week and twice on Sunday

By the way, I changed the wording in the article so people aren't disturbed by the "light gathering ability" phrasing. The new lens allows you to shoot about two stop faster in equivalent light than a 28-50 f2.8 for full frame, with the same depth of field control. Pretty impressive.

Jman13 wrote:
And it's a massively flawed argument that has no real bearing on image production, because the equivalence ONLY holds if the sensor technologies in the two compared cameras are identical. ...

So what part of "all other thigs being equal" didn't you understand? In order to understand a basic principle you have to isolate those factors that are relevant to the principle. Once you have understood the principle, you can start considering what effects changes in other factors will have.

As shown in his post, vieri hasn't understood the principle and you haven't done anything to help him with your post, quite to the contrary.

Let me try to put it another way: Let's take a D800, the Sigma 18-35 f/1.8 and a 27-53/2.8 FF lens. Now we shoot the same scene at the same FOV, wide open. Of course with the DX lens, we are working in crop mode so the resulting image is ~16MP. For the FF lens, we have to raise the ISO value by 1 1/3 stops in order to get the same shutter speed, so noise performance will be 1 1/3 stops worse, right? Wrong. If we downsize the 36 MP image to 16 MP after the fact so the output is identical in size, we will gain 1 1/3 stops of noise performance, just by downsizing. As you can see, you can indeed use the same identical sensor and equivalence works, because the principle works. Of yourse, different sensors will yield different results, but that has nothing to do with the understanding of the principle.

By the way: As long as you don't know the t-stop of the lens, you don't know squat about how much higher shutter speeds you can use. Now is that statement of any help when explaining the effect of numerical aperture (all else being equal!) to someone who doesn't understand it yet?